Investigation of the influence of Ni powder size on microstructural evolution and the thermal explosion combustion synthesis of NiTi

A key microstructural feature that controls the sintering behavior of Ni + Ti powders was determined to be the transformation of alpha-Ti to beta-Ti during heating. The use of very fine Ni powders causes this transformation to occur at the eutectoid temperature (i.e., 765 °C). The use of coarse Ni powders causes a gradual beta-Ti transformation from 765 to 882 °C. At 950 °C a large volume fraction of beta-Ti remains in coarse Ni/Ti mixtures whereas in fine Ni/Ti mixtures this phase is almost eliminated. Further heating above 950 °C causes the beta-Ti to melt, initiating a large exothermic reaction in the coarse Ni/Ti mixtures (i.e., 158 J/g) at 980 °C. The use of fine Ni significantly reduces this reaction (i.e., 3 J/g). Consequently, Ni powder size, and its influence over beta-Ti content can be used to control the reactive sintering behavior of Ni + Ti mixtures.